In this description, the term node (or node equipment) is employed for the intersection point of links in a telecommunications network. A node may be any device or equipment, for example a branching device or a cross-connection device.
In present-day (plesiochronous) telecommunications systems, synchronization may be performed either by means of separate synchronization connections or by utilizing the normal data connections between the system nodes. Separate synchronization connections are used only in isolated cases and very seldom to synchronize an entire network. When data links are used for the synchronization, the line code must be such that the nodes are also capable of recognizing the clock frequency from the incoming data signal. Synchronization of the network nodes from these clock frequencies can be achieved by two basic methods: mutual synchronization and slave synchronization. In mutual synchronization, each node forms its own clock frequency from the mean value of the incoming signal frequencies and its current clock frequency. Hence, all nodes in the network drift towards a common mean frequency and in a steady state have reached said frequency. However, a network employing mutual synchronization cannot be synchronized with a desired source, and thus it will be difficult to interconnect different networks, as in that case the operating frequency of the entire network cannot be precisely determined in advance. In slave synchronization, on the other hand, all network nodes are synchronized with the clock frequency of the master node of the network. Each node selects one incoming signal frequency as the source for its clock frequency. The node seeks to select a signal having the clock frequency of the master node of the network.
In independent slave synchronization, each node makes its decisions about synchronization without receiving any external information to support the decision-making. When the nodes make their decisions on synchronization independently, each node must determine with which node it is synchronized. These determinations are often made in the form of a priority list, and thus the node selects from valid incoming signals the one having the highest priority, i.e. the one highest on the list, as its synchronization source. If this signal is lost or its quality deteriorates so that it is no longer acceptable as a synchronization source, the node selects from the list the signal having the next-highest priority. The priority list must be compiled in such a way that all nodes on the list are located between the node concerned and the master node of the network, and thus synchronization is distributed from the master node to the lower levels.
However, independent slave synchronization poses limitations to network synchronization: in looped networks, all links cannot be used for synchronization, and hence the dynamic adaptability of the network in different situations is limited. Communication must be present between the nodes in order for the information possessed by an individual node to be sufficient for decision-making in all situations without any need to strongly limit the number of links utilized for synchronization, in which case the clock frequency of the master node could not be distributed as easily to the network nodes. There are two methods for such communication, which will be described in the following.
A simple method for expanding independent slave synchronization to be communicative is loop protected synchronization (LP). LP synchronization seeks to prevent the timing from drifting into inoperative state in looped networks by using two state bits mcb and lcb as an aid in the above priority lists, the bits being transmitted between network nodes. The first state bit, the master control bit (mcb), indicates whether the synchronization is derived from the master network node. The master node defined for the network sends this bit as a logical zero in its outgoing signals, and the other nodes relay it further, if they are synchronized with a signal in which the mcb bit has the value zero. The other state bit, the loop control bit (Icb), indicates whether there is a loop in the synchronization. Each node in the network sends this bit as a logical one in the direction in which it is synchronized and as a logical zero in other directions.
Another way in which independent slave synchronization has been expand to be communicative is to use a synchronization status message (SSM) in accordance with the ITU-T standards G.704 and G.708. Standard G.704 defines the frame structure of a digital transmission system operating at a rate 2048 kbit/s. In accordance with the recommendation, bits 4-8 in every second frame are spare bits and may be used e.g. to transport the above synchronization status messages. Only one of bits 4-8 in a frame can be used for this purpose, and thus a four-bit synchronization status message is made up by a selected bit (4-8) in frames 1, 3, 5, and 7 and in frames 9, 11, 13, and 15 of the multiframe. The same synchronization status messages (SSM) are in standard G.708 for SDH networks. In an SDH network, the synchronization status messages are transported in bits b5 . . . b8 of byte S1 in the section overhead (SOH) of the STM-N frame.
The table below presents the synchronization quality levels (QL) indicated by the bit patterns formed by these selected bits San1-San4 (n=4, 5, 6, 7 or 8) S1 (b5 . . . b8). The last column shows the expressions in accordance with to the recommendations.
San1-San4 QL or S1 (b5 . . . b8) Synchronization Quality (QL) Description 0 0000 Quality unknown (Existing Sync. Network) 1 0001 Reserved 2 0010 G.811 3 0011 Reserved 4 0100 G.812 Transit 5 0101 Reserved 6 0110 Reserved 7 0111 Reserved 8 1000 G.812 Local 9 1001 Reserved 10 1010 Reserved 11 1011 Synchronization Equipment Timing Source (SETS) 12 1100 Reserved 13 1101 Reserved 14 1110 Reserved 15 1111 Do not use for Synchronization
As will be seen from the table, ITU-T has decided on four synchronization levels, and additionally a meaning has been given to two further levels; one indicates that the synchronization level is unknown and the other that the signal should not be used for synchronization (QL=1111).
FIGS. 1 and 2 illustrate the operation of the SSM method in a ring-shaped network having five nodes in all, denoted by references N1 . . . N5. Within each node, the quality level of the internal clock of the node (QL:1011) is indicated at the top of the column. Therebeneath the priority list of the node is shown, wherein the selected timing source is indicated in italics. As stated previously, each node selects as its timing source the signal having the highest quality level as indicated by the synchronization message included therein. If several signals have the same quality level, the one highest on the priority list is selected. The synchronization status message transmitted by each node is shown with reference "QL:xxxx" beside each port of the node. External timing sources S1 and S2 are connected to the master node N1 and to node N3 respectively. The quality levels of the synchronization status messages (QL=0010 and QL=0100) sent by the external sources are indicated above the sources. A QL value must be given to each source external of the loop synchronization.
FIG. 1 shows the network in a normal situation (no failures). The master node N1 utilizes an exterior timing source S1, which in this example has been defined to be a clock having the quality level QL=0010. The master node transmits this synchronization status message in both directions. Slave nodes are synchronized with the signal arriving from the port Pa from the main direction; the synchronization status message included in this signal is QL=0010. In this situation, they transmit the same quality level (QL=0010) forward through port Pb and send the quality level QL=1111 (do not use for synchronization) in the direction from which they are receiving their timing (in the direction of port Pa).
FIG. 2 shows a situation in which a failure condition has occurred on the connection between nodes N1 and N2. When node N2 detects this failure, it selects a new timing source. Since it is receiving the quality level QL=1111 from the other direction (from node N3), it cannot use this direction for timing either, and hence it changes to internal timing state and starts transmitting the quality level QL=1011 of its own clock. The next node (node N3) receives this quality level through port Pa, changing external source S2 for its timing source, as the quality level QL=0100 given by this source is higher than that received through port Pa and port Pb cannot be used for timing (QL=1111). Node N3 starts transmitting the quality level QL=0100 in both directions. Node N2 synchronizes itself with the signal arriving from node N3, as the quality level included in that signal is higher than the internal quality level (QL=1011) of node N2, and thus it starts transmitting the quality level QL=1111 in the direction of node N3. Also node N4 accepts the quality level transmitted by node N3, because it is receiving the quality level QL=1111 through port Pb. Hence, node N4 transmits the quality level QL=0100 to node N5, which is synchronized in the direction of port Pb, as the quality level QL=0010 is obtained therefrom. In that situation, node N5 returns the quality level QL=1111 to node N1 and transmits the quality level QL=0010 to node N4. The remaining nodes in the loop do the same, that is, transmit the quality level QL=0010 from port Pa and return the quality level QL=1111 to port Pb. Hence, the situation shown in FIG. 2 has been reached. The loop has thus been synchronized in its secondary direction.
As is apparent from the above examples, the synchronization status messages function well in chain-shaped and ring-shaped networks. On the other hand, in the case of complex network architectures (particularly mesh networks in which more than one path is provided between any two nodes) the synchronization status messages are not capable of preventing synchronization loops from being created. For this reason, the synchronization of complex networks is very difficult to implement in such a way that for example in failure situations no synchronization loops would be created causing the timing to drift towards inoperative state. The synchronization planning of networks has conventionally been carried out manually, which further adds to the possibility of errors.
Furthermore, when conventional planning is used, too long synchronization chains will easily be created in connection with complex networks, as a result of which synchronization sources will be slow to change in failure or altered situations. On account of delays and incorrect use of status messages, also unnecessary changes of synchronization source will easily take place in the network.